1. Field of the Invention
The present invention is directed to a centralizer for the use of maintaining downhole production tubing or the like in a centralized position within the casing and/or hole. The present invention is more specifically designed for use in a blast joint configuration in order to prohibit the blast joint from settling against the casing or any particular side of the hole.
The apparatus of the present invention can be used separate and apart from a blast joint with appropriate retaining ends to prevent slipping. The centralizer would normally be placed over the production tubing of a hydrocarbon well in order to prevent the tubing from settling on one side or the other of the casing. Often wells are drilled in a directional or deviated (nonvertical) manner as opposed to directly vertical and perpendicular to the horizon. In these situations, it has become a concern that the tubing and/or blast joint will rest on the lower end of the casing or at least off center. In light of that concern, the present invention has been developed.
Blast joints are commonly used when an oil or gas well has been drilled which encounters two or more producing formations or zones. In such a situation, each producing formation is produced through a separate string of production tubing extending into the well bore. Typically, a string of production tubing extends to the lowermost producing formation. A packer is set about the production tubing string between the producing formations to isolate the upper producing formation from the lower producing formation. A second string of production tubing extends into the well bore to the upper producing formation. A packer is set above the upper producing formation to close off the annulus about the two strings of production tubing so that the upper production zone is isolated between the two packers. Thus, each string of production tubing is in fluid communication with the production formation adjacent the lower open end of the production tubing. This is commonly referred to as a dual completion well.
Downhole well equipment is exposed to erosive elements in the well bore. This is particularly true in a dual completion well where one string of production tubing extends through an upper producing zone. Flow into the well bore in the upper producing zone, particularly in formations producing high pressure gas, is at high velocities. Abrasive materials, such as unconsolidated sand grains, are often entrained in the fluid stream and impinge on the production tubing. This action is extremely abrasive and erodes the pipe surface, thus requiring replacement of the production tubing. This is a very time consuming process which may be repeated often, particularly of wells having high sand content. Blast joints have been designed and developed in order to protect the tubing.
Many of the blast joints which were used to protect the production tubing utilized tungsten carbide elements. Tungsten carbide is a particularly heavy metal. Even though settling of the production tubing can be a concern without a blast joint, or when a blast joint is utilized which does not include tungsten carbide, the addition of tungsten carbide can cause an even greater concern due to the additional weight which is placed on the tubing.
In a downhole well, particularly when erosive elements are present which require the use of a blast joint, sand and dirt can accumulate around the production tubing and the blast joint if one has been placed on the tubing. Often and dirt cannot be circulated out of the hole because of the sheer amount of the buildup. Eventually, the buildup can become so great the pipe cannot be moved within the casing resulting in stuck pipe. This can be the result of differential wall sticking, the existence of too much sand, dirt and material within a particular confined space, or other similar and related phenomenon.
Normally, in order to alleviate the situation, a wash pipe is run down into the casing to remove the unwanted and undesirable excess dirt, sand and the like. However, if the pipe or blast joint has become so heavy or so off centered, it may be impossible to run the wash pipe through the length of the area which contains the undesirable materials. This is normally due because the pipe or blast joints become stuck or so heavily weighted against one side of the casing, there is simply not enough room for the wash pipe to surround the entire pipe or blast joint in the area where the buildup of undesirable materials has occurred.
The problem with generally accepted and standard centralizers is twofold: they are not normally adapted to be used with a blast joint; and, they may not allow sufficient clearance to be surrounded by a wash pipe. The problem is exacerbated when a blast joint is used because the clearance between the tubing and the casing becomes even more narrow since the circumference of the tubing is surrounded by the protective material of the blast joint. The centralizer must then fit over that material so that the tubing remains protected but still centralized within the casing. In order to overcome this problem, the present invention utilizes a plurality of stress risers which are segmented. This can normally be accomplished by drilling a plurality of stress relief apertures between each vertically segmented portion of each stress relief riser. The segmentation is preferably designed so as to allow sufficient strength to maintain the tubing in a centralized location downhole, but to allow for the stress relief grooves to be sheared when a wash pipe or similar device is utilized to clean out any unwanted buildup of materials or deposits. Once these segments have been sheared off, they can be circulated out of the hole in most situations.
2. Description of the Related Art
U.S. Pat. No. 3,379,269 discloses a system for protecting the production tubing comprising a plurality of baffle sleeves concentrically mounted about the production tubing in the area of an upper producing formation. Each of the sleeves includes perforations which are staggered in relation to the perforations in the next adjacent sleeve so that the erosive fluid entering the well is forced to follow a tortious flow path before it impinges on the production tubing. The changing flow path causes the erosive fluid to decrease its kinetic energy and reduce its impact velocity before it reaches the production tubing, thereby reducing erosion of the tubing.
U.S. Pat. Nos. 4,141,368 and 4,028,796 to Bergstrom disclose a blast joint comprising a series of short cylindrical rings composed of cemented tungsten carbide and the method of producing a blast joint for oil well production tubing. The rings are disposed coaxially in contact with each other between end retaining rings mounted upon a supporting steel tube which comprises a single section or joined sections of production tubing.
In U.S. Pat. No. 4,211,440, Bergstrom suggests that the successful functioning of the blast joint in a well is dependent upon the handling of the blast joint before it is positioned in the well. To this end, Bergstrom discloses the introduction of a yieldable compression spring encircling the production tubing and disposed between the end of the carbide rings and the ring retaining clamp to allow any freedom of movement of the rings relative to the tubing to permit handling and moving of the assembled blast joint without damage to the carbide rings.
U.S. Pat. No. 4,349,050, also to Bergstrom, merely discloses a particular type of end retainer which can be used in conjunction with the blast joints described in the other Bergstrom patents. This patent also discloses and claims certain types of protective coatings for the elements which make up the end retainers.
U.S. Pat. Nos. 4,613,165 and 4,635,968 to Kuhne disclose a multi-joint blast joint. The blast joint of Kuhne is formed by suspending a tubular member having a plurality of rings mounted thereon in the well bore. Pipe slips are used to suspend the tubular member in the well bore. Pipe slips engage the tubular member about an area not covered by the protective rings. A subsequent tubular member is coupled to the tubular member suspended in the well bore and the protective rings are thereafter lowered to enclose the coupling connection.
U.S. Pat. Nos. 4,685,518 and 4,726,423 which are patents assigned to the same assignee as this invention, and which disclosures are incorporated by reference herein, disclose a blast joint which can be easily utilized on downhole tubing which utilizes full strength upset couplings of the tubing joints. This is accomplished by the use of a ring assembly supported in telescoping relation about the erosion resistent rings on one joint said ring assembly being moveable between a first and second position wherein said ring assembly encloses the point of connection between the joints of tubing upon shifting the ring assembly to its second position.
U.S. Pat. No. 4,889,185 discloses a multi-joint blast joint comprising a series of standard length joints production tubing which includes a slip sleeve mounted about the blast joint providing a pipe slip engaging surface for suspending the blast joint in the well bore. Other prior art blast joints include those disclosed in U.S. Pat. Nos. 2,925,097 and 3,365,000 to Duesterberg and Arnwine, respectively.
The present invention is generally designed so as to allow for use with virtually any blast joint where possible.